Low swirl burners (LSBs) have gained popularity in heating and gas power generation industries, in part due to their proven capacity for reducing the production of NO x , which in addition to reacting to form smog and acid rain, plays a central role in the formation of the tropospheric ozone layer. With lean operating conditions, LSBs are susceptible to combustion instability, which can result in flame extinction or equipment failure. Extensive work has been performed to understand the nature of LSB combustion, but scaling trends between laboratory-and industrial-sized burners have not been established. Using hydrogen addition as the primary method of flame stabilization, the current work presents results for a 2.54 cm LSB to investigate potential effects of burner outlet diameter on the nature of flame stability, with focus on flashback and lean blowout conditions. In the lean regime, the onset of instability and flame extinction have been shown to occur at similar equivalence ratios for both the 2.54 cm and a 3.81 cm LSB and depend on the resolution of equivalence ratios incremented. Investigations into flame structures are also performed. Discussion begins with a derivation for properties in a multicomponent gas mixture used to determine the Reynolds number (Re) to develop a condition for turbulent intensity similarity in differently-sized LSBs. Based on this requirement, operating conditions are chosen such that the global Reynolds number for the 2.54 cm LSB is within 2% of the Re for the 3.81 cm burner. With similarity obtained, flame structure investigations focus on flame front curvature and flame surface density (FSD). As flame structure results of the current 2.54 cm LSB work are compared to results for the 3.81 cm LSB, no apparent relationship is shown to exist between burner diameter and the distribution of flame surface density. However, burner diameter is shown to have a definite effect on the flame front curvature. In corresponding flow conditions, a decrease in burner diameter results a broader distribution of curvature and iv an increased average curvature, signifying that compared to the larger 3.81 cm LSB, the flame front of the smaller burner contains tighter, smaller scale wrinkling. v TABLE OF CONTENTS LIST OF TABLES .
Thermoacoustic combustion instability results from the coupling between oscillating heat release and fluctuating pressure inside of a combustion chamber. In the current work, thermoacoustic instability in a low swirl burner is investigated for lean premixed conditions. Measurement of the heat release is a very important aspect of thermoacoustic instability, and in the current experiment the local heat release information is captured with a method based on Planar Laser Induced Fluorescence of the OH radical (OH-PLIF). This is then combined with the pressure signal to quantify the level of thermalacoustic coupling. The specific goal is to examine the global and local flame response to velocity (5 -10 m/s) and driving pressure amplitude (up to 1.12% of atmospheric pressure) changes. The root mean square of a non-dimensional Rayleigh index (R RM S ) was analyzed as the indicator of the global response of flame to acoustic perturbation with different amplitudes. The result shows that the coupling level increases with the forcing amplitude in the beginning. However, when the forcing level is high enough, the coupling saturates. Local response is also examined using a locally-weighted R RM S , focusing on the contribution of the positive and negative coupling regions to the global response. At low velocities, the positive and negative structures play similar roles. However, as velocity is increased, the positive structures become more dominant.
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